To obtain a desired spring characteristic without using an absorber.

A suspension system 25 has a main cantilevered leaf 24, whose free ends are born on the axle 27 of rear wheels 12. On an upper part of each main leaf 24, an upper auxiliary leaf 36 having a smooth projected surface 36A is attached. The main leaf 24 come in contact with the projected surface 36A of the upper auxiliary leaf 36 when deforming flexibly, and the support point of the main leaf 24 change, thus its spring constant change. Accordingly, by properly selecting the shape of the upper auxiliary leaf 36 enables the acquisition of a desired spring characteristic of the main leaf 36, thereby the use of an absorber for the suspension system can be saved to enable a cost reduction and an effective use of spaces.